nnspew builds a sorted database of all available subjects in the
nn article database for fast access via the nngrab utility.
nnspew should be activated regularly to rebuild the subject
database, e.g. by cron. For example:
2 6,9,12,15,18,21 * * * root /bin/nice /usr/lib/nn/nnspew
Cross posted articles are only represented in the database once, and
identical subjects in each group are merged into one to use a minimum
of disk space. This saves about 50% of the disk space otherwise